For two years — since Blake was shot on Sept. 25, 2011 — his mother, Diane Simmons, said she has barely been able to sleep or eat. She quit her job at a grocery store because she could not stand to leave her house. She moved from the home where she and her son had lived.

On Friday, when a Suffolk County jury of seven men and five women convicted Sheldon Mattis and Nyasani Watt, both 20, of first-degree murder, Simmons said she finally began to feel some peace. Her son, a middle school student at the Martin Luther King School in Dorchester, was an innocent casualty of a gang feud in the Bowdoin-Geneva neighborhood of Dorchester.

David L. Ryan/Globe Staff

Sheldon Mattis (left) and Nyasani Watt were part of a gang bent on killing rivals, prosecutors said.

“I can relax,” Simmons said in an interview after the verdict. “I’m so happy, just really happy . . . I’m just glad that justice has been served and that my son and I can be at peace and rest.”

Prosecutors said that Mattis and Watt were part of a gang bent on killing rivals, and had targeted Blake and a 14-year-old friend because the friend lived on a street they considered enemy territory. During 2010 and 2011, three boys and at least one man were targeted in the area by gang members trying to send a message to rivals. They were selected, police said, based solely on geography.

In Blake’s case, prosecutors said Watt and Mattis went after the boys after the 14-year-old told Mattis he lived on Everton Street during a casual conversation outside a Walgreens.

Mattis then left to find Watt and helped him conceal a .40-caliber pistol.

Prosecutors said Mattis patted Watt on the back just before Watt took off on his bike down Geneva Avenue to find the boys.

Watt fired six rounds at Blake and the 14-year-old, who was struck in the neck and shoulder but managed to run away. Blake was shot through his spinal cord and collapsed to the ground, partially paralyzed and gasping for air. His brother rushed to the scene and watched helplessly as paramedics tried to save him. He died two hours later on the operating table at Boston Medical Center.

“People of conscience should be angry about this case,” Suffolk District Attorney Daniel F. Conley said in a prepared statement following the verdict. “They should be angry that two boys, just 14 and 16, would be gunned down on the streets of their own city without provocation. There’s no gang or allegiance or code of conduct that can excuse this act or even make sense of it.”

After the verdict, Simmons hugged her brother and friends who had come to the courtroom to hear the verdict.

Mattis, dressed in a button-down shirt and tie, shook his head and looked back at his mother and father who sat in the courtroom. “I love you,” he mouthed to them as he was led away in handcuffs.

“It’s not over,” Davis told her and patted her back. There is an automatic appeals process on all first-degree murder convictions.

The men are scheduled to be sentenced Dec. 2. The conviction carries an automatic life sentence, but because Watt was a few weeks shy of his 18th birthday at the time of the shooting, a separate sentencing hearing could be held, due to a Supreme Court decision last year that forbids automatic life sentences for juveniles.

Mattis’s lawyer, Kelli Porges, left without speaking to reporters and could not be reached later at her office.

Mattis’s parents insisted their son is innocent and that the jury had failed to see that the case was riddled with inconsistent witnesses and poor evidence. The gun used in the shooting was never recovered.

“We’re losing a son, too,” said Mattis’s mother, Denise, 41. She declined to give her last name.

Mattis’s girlfriend, Ebony Williams, 19, fled the courtroom in tears when the verdict was read. She looked stunned as she sat on a bench outside the courtroom.

“This makes no sense,” Williams said. “How are you convicted for something you didn’t do?”

After the verdict, Simmons went to the Garden of Peace, a memorial for murder victims across from the Superior Courthouse.

She went there every day during the trial, during which she listened to excruciating testimony about her son’s final hours.